1. Field of the Invention
The present invention relates to an Orthogonal Frequency Division Multiple Access/Time Division Duplex (OFDMA/TDD) cellular system, and more specifically, to a method for dynamic resource allocation of uplink and downlink in an OFDMA/TDD cellular system, which is suitable for proposing an uplink and downlink dynamic resource allocation algorithm for enhancing the sector throughput and fairness of an 802.16e OFDMA/TDD cellular system.
2. Description of the Related Art
In general, an OFDM (Orthogonal Frequency Division Multiplexing) scheme is considered to be applied to the 4G mobile communication system because it has high transmission efficiency and supports a simple channel equalization scheme. Further, an OFDMA scheme based on the OFDM scheme is a multiuser access scheme which allocates different sub-carriers to each user. In the OFDMA scheme, resources are allocated in accordance with a user's request, thereby providing various QoS (Quality of Service). The OFDMA scheme is used at the physical layer of IEEE 802.16a and has been adopted as a wireless access scheme of the high-speed mobile Internet which is being actively studied in Korea.
However, to construct an OFDMA-based cellular system, studies on many fields should be continuously conducted. For example, studies on a cell planning method for increasing the coverage of an OFDMA cellular system and a resource allocation algorithm for effectively managing wireless resources to increase a cell capacity need to be conducted. Further, because of a characteristic of the cellular system, effective cell searching and development of synchronization algorithm are essential. In addition, studies on link adaptation, such as modulation level and dynamic channel allocation, and a dynamic modulation scheme are also needed to be conducted.
FIG. 1 is a diagram showing a frame structure of an 802.16e OFDMA TDD system. In FIG. 1, a downlink is composed of 27 symbols, in which the first symbol is used as a preamble, a MAP message header composed of more than two symbols is positioned next to the preamble, and the rest 24 symbols at the maximum are used as a data transfer section. Using the preamble, a base station can maintain synchronization and obtain channel information on sub-channels of each user. An uplink is composed of 15 symbols, in which first three symbols are used as a CQI (Channel Quality Indication) section. The CQI section serves to transfer to the base station the channel information on sub-channels of each user, which is measured using a downlink preamble. The rest 12 symbols are used as a data transfer section.
FIG. 2 is a diagram showing a classification of dynamic resource allocation algorithm. A dynamic resource allocation algorithm is roughly divided into an optimal dynamic resource allocation algorithm and a sub-optimal dynamic resource allocation algorithm. The optimal dynamic resource allocation algorithm is where a sub-channel and proper power, which satisfy a data rate requested by a user, are simultaneously allocated by Lagrange relaxation. The optimal dynamic resource allocation algorithm is not used in an actual system, because the calculation is complicated. The sub-optimal allocation algorithm is where a sub-channel is first allocated to a user and proper power is then allocated in accordance with the sub-channel. The sub-optimal dynamic resource allocation algorithm has an advantage in that an amount of calculation is much smaller than in the optimal dynamic resource allocation algorithm. The sub-optimal dynamic resource allocation algorithm is classified into a transmission power margin dynamic (margin adaptive) algorithm and a data rate dynamic (rate adaptive) algorithm.
FIG. 3 is a diagram showing a classification of the sub-optimal dynamic resource allocation algorithm. The sub-optimal dynamic resource allocation algorithm is roughly classified into a dynamic channel allocation algorithm and a dynamic power allocation algorithm. The dynamic channel allocation algorithm is classified into three algorithms depending on fairness, throughput, and QoS. The dynamic power allocation algorithm is classified into an optimal power allocation algorithm, an EBPU (Equal Band per User) algorithm, and an EBP (Equal Band Power) algorithm.
FIG. 4 is a process of a BABS (Bandwidth Assignment based on the SNR) algorithm which is one of the dynamic channel allocation algorithms. In the BABS algorithm, the following two conditions are assumed. Firstly, a frequency band provided to a system is infinite. Therefore, a sufficient frequency band is present, which can satisfy a data rate requested by each user. Secondly, when a sub-channel is additionally allocated to a user to which a large number of sub-channels are allocated, consumed transmission power is smaller than when a sub-channel is additionally allocated to a user to which a small number of sub-channels are allocated.
In the process, a data rate Rk, which can be provided to the respective users from one sub-channel in a current channel state, is calculated using the average channel value of all sub-channels fed back from the users. Then, the number of sub-channels mk is determined, which can satisfy a data rate Rk, req requested by each of the users.
If the sum of sub-channels mk to be allocated to the respective users exceeds the number of overall sub-channels, the sub-channels of a user to whom the smallest number of sub-channels have been allocated are canceled one by one until the sum of sub-channels mk does not exceed the number of overall sub-channels. This is based on the second condition. Specifically, when the sub-channels of a user to whom a small number of sub-channels are allocated are canceled, a transmission power gain is larger than when the sub-channels of a user to whom a large number of sub-channels are allocated are canceled. If the sum of sub-channels mk to be allocated to the respective users is smaller than the number of overall sub-channels, a sub-channel is additionally allocated to a user, to whom a sub-channel can be allocated with the smallest additional transmission power GK, until the sum of sub-channels mk becomes equal to the number of overall sub-channels. Through such a process, the number of sub-channels to be allocated to each user is calculated.
FIG. 5 is a process of an RCG (Rate Craving Greedy) algorithm. After the number of sub-channels mk to be allocated to each user is determined through the BABS algorithm, the sub-channels are allocated to the user by using channel information. The RCG algorithm is one of the downlink sub-optimal dynamic channel allocation algorithms which allocate subcarriers to users. The RCG algorithm is aimed at maximizing the sum of transmission rates by using a method of estimating transmission rates of users at the respective sub-channels.
The base station allocates a sub-channel to a user who can transmit data at the maximum data rate for each sub-channel. Then, when the number of allocated sub-channels for each user is larger than the required number of sub-channels mk which is previously determined by the BABS algorithm, a user who can minimize the loss of the overall data rate of the system is searched for, among users to whom sub-channels are not allocated by the predetermined number of sub-channels. Further, unallocated sub-channels are searched for. Then, sub-channel re-allocation is performed.
FIG. 6 shows an example of an RCG sub-channel allocation algorithm. The process thereof is performed as follows.
1) It is assumed that the number of sub-channels, which are to be allocated to each user by the BABS algorithm, is set to two.
2) At a first sub-channel, the corresponding sub-channel is allocated to a first user who has the highest channel gain.
3) At a second sub-channel, the corresponding sub-channel is allocated to the first user who has the highest channel gain.
4) From a third sub-channel to the last sub-channel, the above-described process is repeatedly performed.
5) Since the first user has received a larger number of sub-channels than the number of sub-channels to be allocated to the user, two sub-channels should be re-allocated to other users.
6) At this time, to minimize the loss of the entire system, the first and second sub-channels of the first user are re-allocated to a fourth user and a second user, respectively.
7) Since a third user also has received a larger number of sub-channels than the number of sub-channels to be allocated, one sub-channel is re-allocated by the same process.
The RCG algorithm has an advantage in that fairness is guaranteed to all users while the loss of the entire system is minimized. However, since the complex sub-channel allocation should be performed two times, a burden of complexness is present.
FIG. 7 is a diagram showing a first step of a BCS (Best Channel Selection per user considering fairness) algorithm.
The BCS algorithm is an algorithm which considers fairness, in consideration of an actual data rate of a user. The BCS algorithm is composed of two steps.
At the first step shown in FIG. 7, a user is selected, who has the smallest ratio of the sum of data rates allocated up to now to a requested data rate. In FIG. 7, four users require a data rate of 400 kbps. Among them, a first user has acquired a data rate of 100 kbps up to now. Therefore, the first user is selected in consideration of fairness.
FIG. 8 is a diagram showing a second step of the BCS algorithm. In the second step, a sub-channel which guarantees the best data rate is allocated to the selected user. Among the sub-channels of the first user determined by the first step of FIG. 7, a sixth sub-channel has the highest channel gain. Therefore, the base station allocates the sixth sub-channel to the first user, because the sixth sub-channel can guarantee a high data rate.
FIG. 9 is a diagram showing the concept of an FLR (Full Loading Range) algorithm which is a conventional uplink dynamic resource allocation algorithm. In a system using the OFDMA TDD scheme, a terminal can use at least one channel or all channels at the maximum, in order to satisfy various data rates and service levels for each user. Therefore, since the power of a DAC output signal is variable depending on the number of sub-channel NSCH allocated to an uplink, the transmission power of the terminal is determined by the number of sub-channel NSCH and a gain of a power amplifier. Accordingly, in the power control of the OFDMA system, a path loss according to a change in distance between the base station and the terminal and the number of sub-channels NSCH should be considered synthetically.
Further, since the terminal is designed to use smaller transmission power than the base station, limitation of an FLR where the terminal can use all sub-channels should be considered simultaneously. In a terminal within the FLR, the gain of the power amplifier is adjusted in accordance with a distance between the base station and the terminal, and the power of a DAC output signal is proportional to the number of sub-channels NSCH. Therefore, while open-loop power control is used, closed-loop power control can be performed by the same method as a CDMA system.
Outside the FLR, however, although a distance between the base station and a terminal is increased, the gain of the power amplifier is fixed to the maximum value and does not increase any more. Therefore, uplink transmission power changes in accordance with the number of sub-channels NSCH. Further, when the number of sub-channels NSCH is constant, transmission power becomes constant regardless of the distance between the base station and the terminal. Therefore, when an uplink signal transmitted from the outside of the FLR is received by an antenna of the base station, a target SNR (Signal-to-Noise Ratio) cannot be satisfied because power for each sub-channel is insufficient. To solve such a problem, a method of increasing power of the sub-channel is applied.
In the mobile environment, however, a received signal of a terminal is affected by AWGN (Additive White Gaussian Noise), a path loss, multi-path fading, shadowing and the like. Therefore, a standard deviation of 8-12 dB occurs in received power. Because of this, the method of determining an FLR by using a received SNR is very complicated and is not accurate. Further, the number of sub-channels to be allocated to each user and inaccuracy of power control are increased. Accordingly, an interference with other users occurs, and the performance of the system is degraded.